Generalized Bilinear Differential Operators, Binary Bell Polynomials ...

Hindawi Publishing Corporation Abstract and Applied Analysis Volume 2014, Article ID 738609, 6 pages http://dx.doi.org/10.1155/2014/738609

Research Article Generalized Bilinear Differential Operators, Binary Bell Polynomials, and Exact Periodic Wave Solution of Boiti-Leon-Manna-Pempinelli Equation Huanhe Dong,1 Yanfeng Zhang,1 Yongfeng Zhang,1 and Baoshu Yin2,3 1

College of Mathematics and System Science, Shandong University of Science and Technology, Qingdao 266590, China Institute of Oceanology, China Academy of Sciences, Qingdao 266071, China 3 Key Laboratory of Ocean Circulation and Wave, Chinese Academy of Sciences, Qingdao 266071, China 2

Correspondence should be addressed to Yanfeng Zhang; [email protected] Received 5 May 2014; Accepted 20 June 2014; Published 8 July 2014 Academic Editor: Tiecheng Xia Copyright © 2014 Huanhe Dong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We introduce how to obtain the bilinear form and the exact periodic wave solutions of a class of (2 + 1)-dimensional nonlinear integrable differential equations directly and quickly with the help of the generalized 𝐷𝑝 -operators, binary Bell polynomials, and a general Riemann theta function in terms of the Hirota method. As applications, we solve the periodic wave solution of BLMP equation and it can be reduced to soliton solution via asymptotic analysis when the value of 𝑝 is 5.

1. Introduction It is significantly important to research nonlinear evolution equations in exploring physical phenomena in depth [1, 2]. Since the soliton theory has been proposed, the research on seeking the exact solutions of the soliton equations has attracted great attention and made great progress. A series of methods have been proposed, such as Painléve test [3], Bäcklund transformation method [4, 5], Darboux transformation [6], inverse scattering transformation method [7], Lie group method [8, 9], and Hamiltonian method [10, 11]. Particularly, Hirota direct method [12, 13] provides a direct approach to solve a kind of specific bilinear differential equations among the exciting methods. As we all know, once the bilinear forms of nonlinear differential equations are obtained, we can construct the multisoliton solutions, the bilinear Bäcklund transformation, and Lax pairs easily. It is clear that the key of Hirota direct method is to find the bilinear forms of the given differential equations by the Hirota differential 𝐷-operators. Recently, Ma put forward generalized bilinear differential operators named 𝐷𝑝 -operators in

[14] which are used to create bilinear differential equations. Furthermore, different symbols are also used to furnish relations with Bell polynomials in [15], and even for trilinear equations in [16]. In this paper, we would like to explore the relations between multivariate binary Bell polynomials [17–19] and the 𝐷𝑝 -operators and to find the bilinear form of Boiti-LeonManna-Pempinelli (BLMP) equation [20, 21]. Then, we can obtain the exact periodic wave solution [22–25] of the BLMP equation with the help of a general Riemann theta function in terms of Hirota method. The paper is structured as follows. In Section 2, we will give a brief introduction about the difference between the Hirota differential 𝐷-operators and the generalized 𝐷𝑃 operators. In Section 3, we will explore the relations between multivariate binary Bell polynomials and the 𝐷𝑝 -operators. In Section 4, we will use the relation in Section 2 to seek the differential form of the BLMP equation and then take advantage of the Riemann theta function [26, 27] and Hirota method to obtain its exact periodic wave solution which can be reduced to the soliton solution via asymptotic analysis.

2

Abstract and Applied Analysis

2. Hirota Bilinear 𝐷-Operators and the Generalized 𝐷𝑝 -Operators

where, for an integer 𝑘, the 𝑘th power of 𝛼 is defined by

It is known to us that Hirota bilinear 𝐷-operators play a significant role in Hirota direct method. The 𝐷-operators are defined in [14] as the following: 𝐷𝑥𝑚 𝐷𝑡𝑛 𝑎 (𝑥, 𝑡) 𝑏 (𝑥, 𝑡) =

𝜕 𝑚 𝜕𝑛 𝜕𝑦𝑚 𝜕𝑦𝑛

󵄨 × 𝑎(𝑥 + 𝑦, 𝑡 + 𝑠)𝑏(𝑥 − 𝑦, 𝑡 − 𝑠)󵄨󵄨󵄨𝑠=0,𝑦=0 , (1) where 𝑚, 𝑛 = 0, 1, 2, . . .. Generally, we have 𝐷𝑥𝑛 𝐷𝑦𝑚 𝐷𝑡𝑠 𝑎 (𝑥, 𝑦, 𝑡) 𝑏 (𝑥󸀠 , 𝑦󸀠 , 𝑡󸀠 ) =(

𝜕 𝜕 𝑛 𝜕 𝜕 𝑚 − 󸀠) ( − 󸀠) 𝜕𝑥 𝜕𝑥 𝜕𝑦 𝜕𝑦

×(

𝛼 = −1,

𝛼2 = 1,

𝛼3 = −1,

𝛼4 = 𝛼5 = 1,

𝛼6 = −1,

𝛼7 = 1,

𝛼8 = −1,

𝛼9 = 𝛼10 = 1,

𝑢𝑡𝑡 − 𝑢𝑥𝑥 − 3(𝑢 )𝑥𝑥 − 𝑢𝑥𝑥𝑥𝑥 = 0,

(3)

under 𝑢 = 2(ln 𝐹)𝑥𝑥 , we have 3 4 𝐹 − 𝐹𝑥𝑥 𝐹 𝐹2 𝑥

(4)

6 2 𝐹 𝐹 + 4𝐹𝑥 𝐹𝑥𝑥𝑥 − 𝐹𝑥𝑥𝑥𝑥 𝐹 = 0; 𝐹 𝑥 𝑥𝑥

.. .

2 𝐷5,𝑥 𝑓 ⋅ 𝑔 = 𝑓𝑥𝑥 𝑔 − 2𝑓𝑥 𝑔𝑥 + 𝑓𝑔𝑥𝑥 , 4 𝑓 𝐷5,𝑥

Now, under 𝑢 = 2(ln 𝐹)𝑥𝑥 , the generalized bilinear Boussinesq equation can be expressed as 2 2 4 (𝐷5,𝑡 − 𝐷5,𝑥 − 𝐷5,𝑥 ) 𝐹 ⋅ 𝐹 = 0.

(𝐷𝑡2 − 𝐷𝑥2 − 𝐷𝑥4 ) 𝐹 ⋅ 𝐹 = 0.

(5)

However, based on the Hirota 𝐷-operators, Professor Ma put forward a kind of generalized bilinear 𝐷𝑝 -operators in [14]: 𝑛 𝑚 𝑠 𝐷𝑝,𝑦 𝐷𝑝,𝑡 𝑓 (𝑥, 𝑦, 𝑡) 𝑔 (𝑥󸀠 , 𝑦󸀠 , 𝑡󸀠 ) 𝐷𝑝,𝑥

(9)

⋅ 𝑔 = 𝑓4𝑥 − 4𝑓3𝑥 𝑔𝑥 + 6𝑓𝑥𝑥 𝑔𝑥𝑥 − 4𝑓𝑥 𝑔3𝑥 + 𝑓𝑔4𝑥 .

we can get its bilinear form with 𝐷-operators

(10)

Then, we would like to discuss how to use the 𝐷-operator to seek the bilinear differential form of other nonlinear integrable differential equations with the help of binary Bell polynomial.

3. Binary Bell Polynomial

𝜕 𝜕 𝑛 𝜕 𝜕 𝑚 =( + 𝛼 󸀠) ( + 𝛼 󸀠) 𝜕𝑥 𝜕𝑥 𝜕𝑦 𝜕𝑦 󵄨󵄨 𝜕 𝜕 𝑠 󵄨 , + 𝛼 󸀠 ) 𝑓 (𝑥, 𝑦, 𝑡) 𝑔 (𝑥󸀠 , 𝑦󸀠 , 𝑡󸀠 )󵄨󵄨󵄨 󵄨󵄨𝑥󸀠 =𝑥,𝑦󸀠 =𝑦,𝑡󸀠 =𝑡 𝜕𝑡 𝜕𝑡 (6)

As we all know, Bell proposed three kinds of exponentform polynomials. Later, Lambert, Gilson, and their partners generalized the third type of Bell polynomials in [28, 29] which is used mainly in this paper. The multidimensional binary Bell polynomials which we will use are defined as follows:

𝑌𝑛1 𝑥1 ,...,𝑛𝑙 𝑥𝑙 (𝑦) = 𝑌𝑛1 ,...,𝑛𝑙 (𝑦𝑟1 , . . . 𝑦𝑟𝑙 ) −𝑦 𝑛1 𝑛𝑙 𝑦 (𝑛1 , . . . , 𝑛𝑙 ≥ 0) , 󵄨󵄨 = 𝑒 𝜕𝑥1 , . . . , 𝜕𝑥𝑙 𝑒 Y𝑛1 𝑥1 ,...,𝑛𝑙 𝑥𝑙 (𝜐, 𝜔) = 𝑌𝑛1 ,...,𝑛𝑙 (𝑦)󵄨󵄨󵄨(𝑦 𝑟1 +⋅⋅⋅+𝑟𝑙 )𝜐𝑟 𝑥 ,...,𝑟 𝑥 +(1/2)(1−(−1)𝑟1 +⋅⋅⋅+𝑟𝑙 )𝜔 𝑟 𝑥 ,...,𝑟 𝑥 =(1/2)(1+(−1) 1 1

in which 𝑦𝑟1 𝑥1 ,...,𝑟𝑙 𝑥𝑙 = 𝜕𝑥𝑟11 , . . . , 𝜕𝑥𝑟𝑙𝑙 𝑦(𝑥1 , . . . , 𝑥𝑙 ).

(8)

𝐷5,𝑥 𝐷5,𝑡 𝑓 ⋅ 𝑔 = 𝑓𝑥𝑡 𝑔 − 𝑓𝑥 𝑔𝑡 − 𝑓𝑡 𝑔𝑥 + 𝑓𝑔𝑥𝑡 ,

2

×(

(7)

By (6) and (8), it is clear to see that

where 𝑚, 𝑛, 𝑠 = 0, 1, 2, . . .. For instance, for the Boussinesq equation

−

if 𝑘 ≡ 𝑟 (𝑘) mod 𝑝

with 0 ⩽ 𝑟(𝑘) < 𝑝. For example, if 𝑝 = 2𝑘 (𝑘 ∈ 𝑁), the 𝐷-operators are Hirota operators. If 𝑝 = 5, we have

(2)

󵄨󵄨 𝜕 𝜕 𝑠 󵄨 , − 󸀠 ) 𝑎 (𝑥, 𝑦, 𝑡) 𝑏 (𝑥󸀠 , 𝑦󸀠 , 𝑡󸀠 )󵄨󵄨󵄨 󵄨󵄨𝑥󸀠 =𝑥,𝑦󸀠 =𝑦,𝑡󸀠 =𝑡 𝜕𝑡 𝜕𝑡

− 𝐹𝑡2 + 𝐹𝑡𝑡 𝐹 + 𝐹𝑥2 +

𝛼𝑘 = (−1)𝑟 (𝑘) ,

𝑙 𝑙

1 1

1 1

In that way, we have

𝑟1 𝑥1 ,...,𝑟1 𝑥1

)

,

(11)


3

𝑦𝑥 (𝜐, 𝜔) = 𝜐𝑥 , 𝑦2𝑥 (𝜐, 𝜔) =

𝜐𝑥2

When 𝑝 = 5, we can obtain that

+ 𝜔𝑥𝑥 ,

(12)

𝑃5;𝑦,𝑡 = 𝑞𝑥𝑡 ,

𝑦𝑥,𝑦 (𝜐, 𝜔) = 𝜔𝑥𝑦 + 𝜐𝑥 𝜐𝑦 .

𝑃5;2𝑥 = 𝑞𝑥𝑥 , 2 𝑃5;4𝑥 = 3𝑞2𝑥 + 𝑞4𝑥 ,

For convenience, we assume that

(17)

𝑃5;3𝑥,𝑦 = 𝑞3𝑥𝑦 + 3𝑞𝑥𝑥 𝑞𝑥𝑦 , 𝜉(𝑥1 ,...,𝑥𝑙 )

𝐹=𝑒

𝜉=

,

𝜔+𝜐 , 2

𝜂(𝑥1 ,...,𝑥𝑙 )

𝐺=𝑒 𝜂=

, (13)

𝜔−𝜐 2

and read that

In this section, firstly, we will give the bilinear form of BLMP equation with the help of P-polynomials and the 𝐷𝑝 operators. And then, we construct the exact periodic wave solution of BLMP equation with the aid of the Riemann theta function, Hirota direct method, and the special property of the 𝐷𝑝 -operators when acting on exponential functions.

𝑛1 𝑛𝑙 , . . . , 𝐷𝑝,𝑥 𝐹 (𝜉) ⋅ 𝐺 (𝜂) = 𝐺−1 (𝜂) 𝐹−1 (𝜉) 𝐷𝑝,𝑥 1 𝑙 𝑛𝑙

𝑙

𝑛

= ∑ ⋅ ⋅ ⋅ ∑ ∏𝛼𝑘𝑖 (𝑘𝑖 ) (𝑒−𝜉 𝜕𝑥𝑛11 −𝑘1 , . . . , 𝜕𝑥𝑛𝑙𝑙 −𝑘𝑙 𝑒𝜉 ) 𝑘1 =0

𝑖

𝑘𝑙 =0 𝑖=1

4.1. Bilinear Form. BLMP equation can be written as

× (𝑒−𝜂 𝜕𝑥𝑛11 −𝑘1 , . . . , 𝜕𝑥𝑛𝑙𝑙 −𝑘𝑙 𝑒𝜂 ) =

𝑛1

Let us now utilize the P-polynomials given above to seek the bilinear form of BLMP equation with the 𝐷𝑝 -operators.

4. Boiti-Leon-Manna-Pempinelli Equation

𝑛1 𝑛𝑙 , . . . , 𝐷𝑝,𝑥 𝐹⋅𝐺 (𝐹𝐺)−1 𝐷𝑝,𝑥 1 𝑙

𝑛1

𝑃5;2𝑥,𝑡 = 0.

𝑛𝑙

𝑙

𝑛 ∑ ⋅ ⋅ ⋅ ∑ ∏ (𝑘𝑖 ) 𝑌𝑛1 −𝑘1 ,...,𝑛𝑙 −𝑘𝑙 𝑖 𝑘1 =0 𝑘𝑙 =0 𝑖=1

(𝜉) , 𝑌𝑘1 ,...,𝑘𝑙 (𝛼

𝑢𝑦,𝑡 + 𝑢3𝑥,𝑦 − 3𝑢2𝑥 𝑢𝑦 − 3𝑢𝑥 𝑢𝑥,𝑦 = 0. 𝑟

∑𝑟𝑙1 𝑟𝑖

𝜂)

Setting 𝑢 = −𝑞𝑥 , inserting it into (18), and integrating with respect to 𝑥 yields

𝑟𝑙

= 𝑌𝑛1 ,...,𝑛𝑙 (𝑦𝑟1 ,...,𝑟𝑙 = 𝜉𝑟1 ,...,𝑟𝑙 + 𝛼∑𝑟1 𝑟𝑖 𝜂𝑟1 ,...,𝑟𝑙 ) = 𝑌𝑛1 ,...,𝑛𝑙 (𝑦𝑟1 ,...,𝑟𝑙 =

𝑞𝑦,𝑡 + 𝑞3𝑥,𝑦 + 3𝑞𝑥𝑥 𝑞𝑥,𝑦 − 𝜆 = 0,

𝑟𝑙 1 (1 + 𝛼∑𝑟1 𝑟𝑖 ) 𝜐𝑟1 ,...,𝑟𝑙 2

𝑃5;𝑦,𝑡 (𝑞) + 𝑃5;3𝑥,𝑦 (𝑞) − 𝜆 = 0.

(14) We find that the link between Y-polynomials and the 𝐷𝑝 -operator can be given as the following through the above deduction: ln 𝐹 , 𝜔 = ln 𝐹𝐺) 𝐺

= 𝑌𝑛1 ,...,𝑛𝑙 (𝑦𝑟1 ,...,𝑟𝑙

(20)

From the above, we can get the bilinear form of (18):

= Y𝑝;𝑛1 𝑥1 ,...,𝑛𝑙 𝑥𝑙 (𝜐, 𝜔) .

𝑟𝑙 1 = (1 + 𝛼∑𝑟1 𝑟𝑖 ) 𝜐𝑟1 , . . . , 𝑟𝑙 2

(19)

where 𝜆 is an integral constant. Based on (17), (19) can be expressed as

𝑟𝑙 1 + (1 + 𝛼∑𝑟1 𝑟𝑖 ) 𝜔𝑟1 ,...,𝑟𝑙 ) 2

Y𝑝;𝑛1 𝑥1 ,...,𝑛𝑙 𝑥𝑙 (𝜐 =

(18)

3 (𝐷5,𝑦 𝐷5,𝑡 + 𝐷5,𝑥 𝐷5,𝑦 ) 𝐹 ⋅ 𝐹 − 𝜆 ⋅ 𝐹2 = 0

with 𝑞 = 2 ln 𝐹. 4.2. Periodic Wave Solutions. When acting on exponential functions, we find that 𝐷𝑝 -operators have a good property 𝐻 (𝐷𝑝,𝑥1 , . . . , 𝐷𝑝,𝑥𝑙 ) 𝑒𝜉1 ⋅ 𝑒𝜉2

(15)

𝑟𝑙 1 + (1 + 𝛼∑𝑟1 𝑟𝑖 ) 𝜔𝑟1 , . . . , 𝑟𝑙 ) . 2

Particularly, when 𝐹 = 𝐺, we define P-polynomials by P𝑝;𝑛1 𝑥1 ,...,𝑛𝑙 𝑥𝑙 (𝑞) = Y𝑝;𝑛1 𝑥1 ,...,𝑛𝑙 𝑥𝑙 (𝜐 = 0, 𝜔 = 2 ln 𝐹 = 𝑞) . (16)

(21)

= 𝐻 (𝑘1 + 𝛼𝑘2 , 𝑙1 + 𝛼𝑙2 , 𝑤1 + 𝛼𝑤2 ) 𝑒𝜉1 +𝜉2 ,

(22)

if we assume that 𝜉𝑖 = 𝑘𝑖 𝑥 + 𝑙𝑖 𝑦 + 𝑤𝑖 𝑡 + 𝜉𝑖(0)

𝑖 = 1, 2, . . . .

(23)

As a result of the property above, we consider Riemann’s theta function solution of (18): ∞

2

𝐹 = ∑ 𝑒2𝜋𝑖𝑛𝜂+𝜋𝑖𝑛 𝜏 , 𝑛=−∞

(24)

4


where 𝑛 ∈ 𝑍, 𝜏 ∈ 𝐶, Im 𝜏 > 0, 𝜂 = 𝑘𝑥 + 𝑙𝑦 + 𝑤𝑡, with 𝑘, 𝑙, and 𝑤 being constants to be determined. Then, we have 𝐻 (𝐷𝑝,𝑥 , 𝐷𝑝,𝑦 , 𝐷𝑝,𝑡 ) 𝐹 ⋅ 𝐹 2

2

∑ 𝐻 (𝐷𝑝,𝑥 , 𝐷𝑝,𝑦 , 𝐷𝑝,𝑡 ) 𝑒2𝜋𝑖𝑛𝜂+𝜋𝑖𝑛 𝜏 𝑒2𝜋𝑖𝑚𝜂+𝜋𝑖𝑚 𝜏

= ∑

𝑛=−∞ 𝑚=−∞

= ∑

(27)

∞

𝐻 (0) = ∑ {[2𝜋𝑖 (1 − 𝛼) 𝑛]2 𝑙 ⋅ 𝑤 + [2𝜋𝑖 (1 − 𝛼) 𝑛𝑘]3 𝑛=−∞

∞

∞

𝐻 (0) 𝑒𝜋𝑖𝑛𝑞𝜏 , 𝑞 = 2𝑛; 𝐻 (𝑞) = { 𝜋𝑖(2𝑛+2𝑛2 )(𝑞+1)𝜏 𝐻 (1) 𝑒 , 𝑞 = 2𝑛 + 1. For (21), we may let

∞

∞

From (26) we can infer that

2

⋅ 2𝜋𝑖 (1 − 𝛼) 𝑛𝑙 − 𝜆} 𝑒2𝜋𝑖𝑛 𝜏

∑ 𝐻 (2𝜋𝑖 (𝑛 + 𝛼𝑚) 𝑘, 2𝜋𝑖 (𝑛 + 𝛼𝑚) 𝑙,

𝑛=−∞ 𝑚=−∞

2

2

2𝜋𝑖 (𝑛 + 𝛼𝑚) 𝑤) 𝑒2𝜋𝑖(𝑛+𝑚)𝜂+𝜋𝑖(𝑛 +𝑚 )𝜏 ∞

∞

𝑞=−∞

𝑚=−∞

2

∞

3

= ∑ (−16𝑛2 𝜋2 𝑙𝑤 + 25𝑛4 𝜋4 𝑘3 𝑙 − 𝜆) 𝑒2𝜋𝑖𝑛 𝜏 𝑛=−∞

= 0,

= ∑ { ∑ 𝐻 ((2𝜋𝑖 (1 − 𝛼) 𝑛 − 𝛼 𝑞) 𝑘,

∞

𝐻 (1) = ∑ {2𝜋 (𝑖 (1 − 𝛼) 𝑛 − 𝛼2 ) 𝑙

(2𝜋𝑖 (1 − 𝛼) 𝑛 − 𝛼2 𝑞) 𝑙,

𝑛=−∞

⋅ 2𝜋 (𝑖 (1 − 𝛼) 𝑛 − 𝛼2 ) 𝑤

(2𝜋𝑖 (1 − 𝛼) 𝑛 − 𝛼2 𝑞) 𝑤)

3

+ [2𝜋 (𝑖 (1 − 𝛼) 𝑛 − 𝛼2 ) 𝑘]

2

2

× 𝑒𝜋𝑖(𝑛 +(𝑛+𝛼𝑞) )𝜏 } 𝑒2𝜋𝑖(−𝛼𝑞)𝜂

⋅ 2𝜋 (𝑖 (1 − 𝛼) 𝑛 − 𝛼2 ) 𝑙 − 𝜆}

∞

= ∑ 𝐻 (𝑞) 𝑒2𝜋𝑖(−𝛼𝑞)𝜂 ,

2

× 𝑒2𝜋𝑖(𝑛 −2𝑛+1)𝜏

𝑞=−∞

(25) where 𝑞 = −(1/𝛼)(𝑚 + 𝑛). To the bilinear form of BLMP equation, 𝐻(𝑞) satisfies the period characters when 𝑝 = 5. The powers of 𝛼 obey rule (7), noting that ∞

∞

= ∑ [−4(2𝑛 − 1)2 𝜋2 𝑙𝑤 + 16(2𝑛 − 1)4 𝜋4 𝑘3 𝑙 − 𝜆] 𝑛=−∞

2

× 𝑒𝜋𝑖(2𝑛 −2𝑛+1)𝜏 = 0. (28) Also, the powers of 𝛼 obey rule (7). For the sake of computational convenience, we denote that

2

𝐻 (𝑞) = ∑ 𝐻 ((2𝜋𝑖 (1 − 𝛼) 𝑛 − 𝛼 𝑞) 𝑘, 𝑛=−∞

2

𝑔1 (𝑛) = 𝑒2𝜋𝑖𝑛 𝜏 ,

2

(2𝜋𝑖 (1 − 𝛼) 𝑛 − 𝛼 𝑞) 𝑙,

∞

𝑎11 = ∑ − 16𝑛2 𝜋2 𝑙𝑔1 (𝑛) ,

𝜋𝑖(𝑛2 +(𝑛+𝛼𝑞)2 )𝜏

2

(2𝜋𝑖 (1 − 𝛼) 𝑛 − 𝛼 𝑞) 𝑤) 𝑒

𝑛=−∞ ∞

∞

𝑎12 = ∑ (256𝑛4 𝜋4 𝑘3 𝑙) 𝑔1 (𝑛) ,

= ∑ 𝐻 (2𝜋𝑖 (2𝑛 − 𝑞) 𝑘, 2𝜋𝑖 (2𝑛 − 𝑞) 𝑙,

𝑛=−∞

𝑛=−∞

2

∞

2

2𝜋𝑖 (2𝑛 − 𝑞) 𝑤) 𝑒𝜋𝑖(𝑛 +(𝑞−𝑛) )𝜏

𝑎13 = ∑ 𝑔1 (𝑛) ;

∞

𝑛=−∞

ℎ=−∞

𝑔2 (𝑛) = 𝑒𝜋𝑖(2𝑛 −2𝑛+1)𝜏 ,

= ∑ 𝐻 (2𝜋𝑖 (2ℎ − (𝑞 − 2)) 𝑘,

2

2𝜋𝑖 (2ℎ − (𝑞 − 2)) 𝑙, 2𝜋𝑖 (2ℎ − (𝑞 − 2)) 𝑤) 2

𝑛=−∞

2

⋅ 𝑒𝜋𝑖(ℎ +(𝑞−ℎ−2) )𝜏 ⋅ 𝑒2𝜋𝑖(𝑞−1)𝜏 2𝜋𝑖(𝑞−1)𝜏

= 𝐻 (𝑞 − 2) 𝑒

∞

𝑎22 = ∑ (16(2𝑛 − 1)4 𝜋4 𝑘3 𝑙) 𝑔2 (𝑛) ,

, (26)

where ℎ = 𝑛 + 𝛼.

∞

𝑎21 = ∑ − 4(2𝑛 − 1)2 𝜋2 𝑙𝑔2 (𝑛) ,

𝑛=−∞ ∞

𝑎23 = ∑ 𝑔2 (𝑛) . 𝑛=−∞

(29)


5

By (28), (29), and (30), we can get that 𝑎11 𝜔 + 𝑎12 − 𝜆𝑎13 = 0,

In view of (30), it is easy to see that 𝜔= 𝜆=

𝑎13 𝑎22 − 𝑎23 𝑎12 , 𝑎11 𝑎23 − 𝑎13 𝑎21

(31)

𝑎12 𝑎21 − 𝑎11 𝑎22 . 𝑎11 𝑎23 − 𝑎13 𝑎21

F(x, y, t = 10)

(30)

𝑎21 𝜔 + 𝑎22 − 𝜆𝑎23 = 0.

1.05 1 0.95 −100

100

x

Thus, we obtain the periodic wave solution of BLMP equation: 𝑢 = −2(ln 𝐹)𝑥 ,

2

𝑎11 = ∑ − 16𝑛2 𝜋2 𝑙 ⋅ 𝑒2𝜋𝑖𝑛 𝜏 2

8

󳨀→ 1 + 𝑒𝜂1

= 2 × 256𝜋4 𝑘3 𝑙 (𝛾2 + 4𝛾8 + 9𝛾18 + ⋅ ⋅ ⋅ ) , 2

𝑛=−∞

(33) 𝜋𝑖(2𝑛2 −2𝑛+1)𝜏

𝑎21 = ∑ − 4(2𝑛 − 1) 𝜋 𝑙 ⋅ 𝑒

2

𝑎22 = ∑ 16(2𝑛 − 1)4 𝜋4 𝑘3 𝑙 ⋅ 𝑒𝜋𝑖(2𝑛 −2𝑛+1)𝜏

2 3

𝐹 = 1 + 𝑒2𝜋𝑖(𝑘𝑥+𝑙𝑦+4𝜋 𝑘 𝑡+𝜏) ,

𝑛=−∞

= 32𝜋4 𝑘3 𝑙 (𝛾 + 34 𝛾5 + 54 𝛾13 + ⋅ ⋅ ⋅ ) , 2

5. Conclusions and Remarks

𝑛=−∞

which lead to 𝑎11 𝑎23 − 𝑎21 𝑎13 = 8𝜋2 𝑙𝛾 + 𝑜 (𝛾) , (34)

𝑎11 𝑎22 − 𝑎12 𝑎21 = 𝑜 (𝛾) . So, we have 𝜔 → 4𝜋2 𝑘3 and 𝜆 → 0, as 𝛾 → 0. It is interesting that if we set 𝑙1 = 2𝜋𝑖𝑙,

𝑤1 = 2𝜋𝑖𝑤,

𝜂1 = 𝑘1 𝑥 + 𝑙1 𝑦 + 𝑤1 𝑡 + 𝜋𝑖𝜏,

(38)

solution of (18) can be shown in Figure 1.

𝑎23 = ∑ 𝑒𝜋𝑖(2𝑛 −2𝑛+1)𝜏 = 1 + 2𝛾 + 2𝛾5 + 2𝛾13 + ⋅ ⋅ ⋅

𝑘1 = 2𝜋𝑖𝑘,

(37)

Thus, if we assume that 𝑘 = 0.01, 𝑙 = 0.01, and 𝜏 = 𝑖 to the solution

= −8𝜋2 𝑙 (𝛾 + 9𝛾5 + 25𝛾13 + ⋅ ⋅ ⋅ ) ,

𝑎13 𝑎22 − 𝑎12 𝑎23 = 32𝜋4 𝑘3 𝑙𝛾 + 𝑜 (𝛾) ,

From all the above, it can be proved that the periodic wave solution (32) just goes to the soliton solution 𝑤1 = 2𝜋𝑖𝑤 󳨀→ −𝑘13 .

𝑛=−∞

∞

(𝜂1 󳨀→ 0) . (36)

𝑎13 = ∑ 𝑒2𝜋𝑖𝑛 𝜏 = 1 + 2𝛾2 + 2𝛾8 + 2𝛾18 + ⋅ ⋅ ⋅ ,

∞

2

= 1 + 𝑒𝜂1 + 𝛾2 (𝑒𝜂1 + 𝑒2𝜂1 ) + 𝛾6 (𝑒−2𝜂1 + 𝑒3𝜂1 ) + ⋅ ⋅ ⋅ ,

𝑛=−∞

2 2

which can infer that

= 1 + 𝑒𝜋𝑖𝜂 (𝑒2𝜋𝑖𝜂 + 𝑒−2𝜋𝑖𝜂 ) + 𝑒4𝜋𝑖𝜂 (𝑒4𝜋𝑖𝜂 + 𝑒−4𝜋𝑖𝜂 ) + ⋅ ⋅ ⋅ ,

2

∞

y

𝑛=−∞

18

𝑎12 = ∑ 256𝑛4 𝜋4 𝑘3 𝑙 ⋅ 𝑒2𝜋𝑖𝑛 𝜏

∞

−50

Figure 1: Solution of (18).

∞

= −32𝜋 𝑙 (𝛾 + 4𝛾 + 9𝛾 + ⋅ ⋅ ⋅ ) , ∞

0 50

𝐹 = ∑ 𝑒2𝜋𝑖𝜂+𝜋𝑖𝑛 𝜏 ,

𝑛=−∞

2

0 100 −100

(32)

where 𝐹 is given by (24) and 𝜔, 𝜆 are satisfied with (31). Then, assuming 𝑒𝜋𝑖𝜏 = 𝛾, based on (29), we may obtain that ∞

50

−50

(35)

In this paper, we obtain the bilinear form of bilinear differential equations by applying the 𝐷𝑝 -operators and binary Bell polynomials, which has proved to be a quick and direct method. Furthermore, together with Riemann theta function and Hirota method, we successfully get the exact periodic wave solution and figure of BLMP equation when 𝑝 = 5. There are many other interesting questions on bilinear differential equations, for example, how to apply the generalized operators into the discrete equations; it is known that researches on the discrete and differential equations are also significant. Besides, we will try to explore other operators to construct more nonlinear evolution equations simply and directly in the near future.

6

Conflict of Interests The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments This work was supported by National Natural Science Foundation of China (no. 11271007), Nature Science Foundation of Shandong Province of China (no. ZR2013AQ017), the Strategic Pioneering Program of Chinese Academy of Sciences (no. XDA 10020104), SDUST Research Fund (no. 2012 KYTD105), Open Fund of the Key Laboratory of Ocean Circulation and Waves, Chinese Academy of Science (no. KLOCAW1401), and Graduate Innovation Foundation from Shandong University of Science and Technology (no. YC130321).

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